1. Field of Invention
This invention relates generally to a liquid ink printing apparatus and a method for gray scale printing using different size drop ejectors.
2. Description of Related Art
Fluid ejector systems, such as drop-on-demand liquid ink printers, such as piezoelectric, acoustic, phase change wax-based or thermal, have at least one fluid ejector from which droplets of fluid are ejected towards a receiving sheet. Within the fluid ejector, the fluid is contained in a plurality of channels. Power pulses cause the droplets of fluid to be expelled as required from orifices or nozzles at the end of the channels.
In a thermal fluid ejection system, the power pulse is usually produced by a heater transducer or resistor, typically associated with one of the channels. Each resistor is individually addressable to heat and vaporize fluid in one of the channels. As voltage is applied across a selected heater resistor, a vapor bubble grows in the associated channel and initially bulges from the channel orifice followed by a collapse of the bubble. The fluid within the channel then retracts and separates from the bulging fluid to form a fluid droplet moving in a direction away from the channel orifice and towards the recording medium. When the fluid droplet hits the receiving medium, the fluid droplet forms a dot or spot of fluid on the receiving medium. The channel is then refilled by capillary action, which, in turn, draws fluid from a supply container of fluid.
A fluid ejector can include one or more thermal fluid ejector dies having a heater portion and a channel portion. The channel portion includes an array of fluid channels that bring fluid into contact with the resistive heaters, which are correspondingly arranged on the heater portion. In addition, the heater portion may also have integrated addressing electronics and driver transistors. Since the array of channels in a single die assembly is not sufficient to cover the length of a page, the fluid ejector is either scanned across the page with the receiving medium advanced between scans or multiple die assemblies are butted together to produce a full-width fluid ejector.
Because thermal fluid ejector nozzles typically produce spots or dots of a single size, high quality fluid ejection requires the fluid channels and corresponding heaters to be fabricated at a high resolution, such as, for example, on the order of 400-600 or more channels per inch.
When the fluid ejector is an ink jet printhead, the fluid ejector may be incorporated into for example, a carriage-type printer, a partial width array-type printer, or a page-width type printer. The carriage-type printer typically has a relatively small fluid ejector containing the ink channels and nozzles. The fluid ejector can be sealingly attached to a disposable fluid supply cartridge. The combined fluid ejector and cartridge assembly is attached to a carriage that is reciprocated to print one swath of information at a time, on a stationary receiving medium, such as paper or a transparency, where each swath of information is equal to the length of a column of nozzles.
After the swath is printed, the receiving medium is stepped a distance at most equal to the height of the printed swath so that the next printed swath is contiguous or overlaps with the previously printed swath. This procedure is repeated until the entire image is printed.
In contrast, the page-width printer includes a stationary fluid ejector having a length sufficient to print across the width or length of the sheet of receiving medium. The receiving medium is continually moved past the page-width fluid ejector in a direction substantially normal to the fluid ejector length and at a constant or varying speed during the printing process. A page width fluid ejector printer is described, for instance, in U.S. Pat. No. 5,192,959, incorporated herein by reference in its entirety.
Fluid ejection systems typically eject fluid drops based on information received from an information output device, such as a personal computer. Typically, this received information is in the form of a raster, such as, for example a full page bitmap or in the form of an image written in a page description language. The raster includes a series of scan lines comprising bits representing individual information elements. Each scan line contains information sufficient to eject a single line of fluid droplets across the receiving medium a linear fashion. For example, fluid ejecting printers can print bitmap information as received or can print an image written in the page description language once it is converted to a bitmap of pixel information.
In a fluid ejection system having a fluid ejection with an array of equally sized and spaced nozzles, each of the equally sized nozzles produces fluid spots of the same size, and the pixels are placed on a square first grid having a size S. As shown on FIG. 1, the spacing between the centers 62 of the fluid spots in the X and Y direction is equal to S, as illustrated in a sample printing pattern shown in FIG. 1. To create the pattern shown in FIG. 1, nozzles 60, which are schematically represented as triangles, traverse across a receiving medium 69 in the scan direction X. The nozzles 60, which are spaced from one another a specified distance or pitch d on the fluid ejection, deposit the fluid spots or drops on the pixel centers 62. It should be appreciated that the grid spacing in the nozzle array direction Y is perpendicular to the scan direction. In general, to deposit the fluid spots on the receiving medium in a square grid, the pitch d is equal to the grid spacing S.
Typically, the nozzles 60 and the ejection parameters are designed to produce spot diameters of approximately 1.414S (i.e., S2). This allows the space within an solid region of the pattern to be completely filled, by having diagonally adjacent spots touch. A disadvantage of this ejection scheme is that xe2x80x9cjaggednessxe2x80x9d may be objectionable at edges in the pattern, particularly for lines or curves at small angles to the scan direction as illustrated in FIG. 1. In FIG. 1, a first ellipse 64 located outside a second ellipse 66 indicate at what portions of the printed image the jaggedness would be most objectionable. In addition, pattern quality can be determined by 1) how much open space remains within the ring defined by the first and second ellipses 64 and 66, 2) how far the spots 60 extend outside either the first and/or second ellipses 64 and/or 66, and 3) the amount of fluid deposited on the receiving medium.
One technique for improving the edge quality of the pattern is to extend the addressability of the carriage to allow dot placement at intermediate positions in the grid along the scan direction X. It is also possible to improve edge quality of the pattern by increasing the resolution. This, however, increases the complexity and cost of fabrication and typically slows down forming the pattern because of the additional number of spots to be ejected.
The fluid ejection and ejection methods discussed above and illustrated in FIG. 1, for example, provide for forming patterns of ejected fluid drops having sufficient quality, especially when the resolution is increased upwards to and beyond 600 spots per inch. These fluid ejectors and methods, however, do not always provide patterns having the desired quality especially when considering fluid density levels, fluid saving modes, and patterns forming throughput.
A majority of thermal fluid ejection systems produce spots or drops of fluid all having substantially the same diameter, and allow spot size to be controllably varied by at most approximately 10%. Therefore, these conventional fluid ejection systems are not capable of forming a pattern using variable fluid density regions. In thermal fluid ejection systems, for example, drop volume or spot size is determined by many factors, including the heater transducer area, the cross-sectional area of the fluid ejecting channel or nozzle, the pulsing conditions necessary to create a fluid droplet and the physical properties of the fluid itself, such as the temperature of the fluid in the channels. Although spot diameter changes of approximately xc2x110 percent are possible by changing pulsing conditions or fluid temperature during forming the pattern, the given spot size is nominally constant to the extent that deliberate spot size variations cannot span a large enough range to be useful in forming patterns having a variable fluid density.
Another technique for improving pattern forming quality, especially variable density pattern forming quality, uses groups of differently-sized nozzles with a major grid of large spots offset diagonally by 0.5S in the X and Y direction from a minor grid of small spots, where S is the grid spacing. This technique is disclosed in detail in U.S. Pat. No. 5,745,131 to Kneezel et al., incorporated herein by reference in its entirety.
FIG. 2 illustrates a pattern according to the 131 patent, where the pattern is formed with a fluid ejector having a first plurality of nozzles 67 and a second plurality of nozzles 68. The pitch between the individual nozzles 67 is equal to the distance S. The spacing between individual nozzles 68 is also equal to the distance S. However, the first plurality of nozzles 67 is offset from the second plurality of nozzles 68 by 1.5S.
The fluid ejection system fires the individual nozzles 67 and 68 so that the fluid drops land on the grid points 67a and 68a, respectively, in the scan direction. A somewhat better fill is achieved by this techniques compared to the pattern illustrated in FIG. 1, at least in terms of the amount of fluid used. Since the number of nozzles within each of the first plurality of nozzles 67 and the second plurality of nozzles 68 are equivalent, the receiving medium is advanced half the fluid ejection length to achieve proper fill.
Another technique for improving printing quality is disclosed in U.S. Pat. No. 5,598,191 to Kneezel, incorporated herein by reference in its entirety. The 191 patent describes a printhead having first and second linear arrays of ejectors. These ejectors are spaced within each array by a predetermined pitch. The arrays spaced from each other by an integral number of pitches plus a partial pitch. This allows interleaving of print swaths by the two sets of ejectors, in order to print at higher resolution than the predetermined pitch would allow.
Another technique for improving pattern forming quality, especially variable density pattern forming quality, is disclosed in pending U.S. patent application Ser. No. 09/233,110 to Kneezel et al., incorporated herein by reference in its entirety. The 110 application discloses that the optimum spot diameters to reduce the amount of fluid used for the printing pattern shown in FIG. 2 includes a first plurality of nozzles that produce a first spot of fluid having a diameter of 1.250.5 S (=1.12S), and a second plurality of nozzles that produce a second spot of fluid having a diameter of 0.5S. In this case, only 75% of the fluid is required relative to the printing pattern where all of the spots have the same diameter of 1.414S. The 110 application also discloses an alternative printhead configuration which may be used to print the pattern shown in FIG. 2. In this alternative configuration, the nozzles of the first plurality of nozzles are interleaved with the nozzles of the second plurality of nozzles, where the nozzles of each array are spaced at a pitch of S, and the arrays are offset relative to each other at 0.5S. To provide the offset along the scan direction X, the nozzles of the first plurality of nozzles are fired first, followed by the nozzles of the second plurality of nozzles.
The 131 patent also describes a configuration that uses three different sized nozzles with three different offsets along the Y direction. A plurality of each differently-sized nozzle is provided. Spots printed by the pluralities of the two smaller-sized nozzles are offset from the spots printed by the plurality of the largest nozzles along the scan direction X by 0.5S.
This invention provides systems and methods for forming variable density patterns using multiple spots within the grid spacing using at least one of the array of smaller drop ejectors.
The invention separately provides at least some smaller spots which are not relatively offset from the major grid of spots in the Y direction.
In various exemplary embodiments of the systems and methods for variable density pattern forming according to this invention, variable density pattern forming is achieved by producing a plurality of large, medium and small spots. The plurality of large, medium and small spots are produced by a plurality of large, medium and small nozzles, each having a predetermined nozzle diameter.
Furthermore, the plurality of large, medium and small spots are placed on a grid. The grid has a spacing such that the desired amount of ink coverage is achieved. The grid is filled by sequentially ejecting a plurality of large, medium and small spots onto the grid.
In various exemplary embodiments of the systems and methods according to this invention, by appropriately selecting the drops sizes and relative grid spacing for the differently sized fluid drops, an increased number of density levels can be obtained. In various exemplary embodiments of the systems and methods according to this invention, the different density levels are on a generally smooth curve between the minimum and maximum density levels. In various exemplary embodiments, this substantially smooth curve is a substantially straight line.
These and other features and advantages of this invention are described and are apparent from the detailed description of various exemplary embodiments of the systems and methods according to this invention.